Field of the Invention
The present invention relates to a solid state relay, in particular, to a solid state relay and control method for three-phase AC motor running direction.
Brief Description of the Related Art
A three-phase solid state relay in a three-phase motor is used as switches. In general, two groups of power components in the three-phase solid state relay are used for the three-phase power supply and three-phase AC motor reversing switching.
During use, problems of the three-phase AC motor power supply can cause serious consequences.
For example, the phase sequence of the three-phase AC motor changes due to wiring and repairing, which will make the motor work reverse and influence normal operation, will also sometimes damage the external mechanical parts of the motor.
In another example, the three-phase power supply can have phase lack due to wiring, repairing, and vibration, which will make the motor current leap due to phase lack. If the motor works long-term in a state of phase lack, it will make the motor burn down and the solid state relay will be damaged by the high current.
FIG. 1 shows the main structural diagram for existing technology in the solid state relay (SSR) 50, using discrete components. The three-phase solid state relay (SSR) 50 includes a driver module 51 and a power components module 52 which is connected to the driver module 51. Among them, the input terminal I1 and I2 of the driver module 51 are connected to the control signal CS. The control signal CS inputs DC control signal in input terminal I1 or I2. The input terminal L1,L2,L3 of the power components module 52 are connected to the three-phase power supply, the output terminals U,V,W are connected to the load LOAD. LOAD is mainly composed of a motor. The working principle is that, when the l1 is valid, the output terminal U is connected to input terminal L1, the output terminal V is connected to input terminal L2, and the output terminal W is connected to input terminal L3. When the l2 is valid, the output terminal U is connected to input terminal L1, the output terminal V is connected to input terminal L3, and the output terminal W is connected to input terminal L2. This can be controlled via the l1 and l2 to change the phase sequence of the three-phase power supply added at the load end, thereby controlling the motor running direction.
The thyristor is usually used as the power component in power components module 52, while the driver module 51 is usually composed of triode, resistance, capacitance, and photoelectric couplers. The constant current circuit, which is composed of triode, resistance, and capacitance, receives the control signal of input terminal l1 or l2, so that the light emitting diode in the photoelectric coupler could luminescence and make the output of the photoelectric coupler enter a conducting state and trigger the corresponding thyristor conduction.
Because the thyristor turn-off characteristics are relatively special, the load current must be less than a certain degree to completely shut off the thyristor. Therefore, in switching the two groups of power components in the solid state relay itself for a short time, one group of power components are not completely cut-off while another group of power components are conducted, leading to internal interphase short circuit of the three-phase solid state relay, which can then damage the equipment.
At present, in using the external equipment (such as phase sequence protector and controller MCU) to detect the three-phase power supply phase sequence and phase lacking state, according to the power supply state, the external equipment provides the control signal to the solid state relay, and then controls the power components' turn-on and turn-off time to solve the above problems.
Although the use of external equipment could solve some problems in the use of solid state relay for three-phase AC motor, the volume of the external equipment itself is larger, the system is more complex (For example, in order to process the signal logical relations, the central processor will be required), the price is higher, and there are also compatibility problems and obvious shortcomings. So, we must have a kind of structure function which is centralized, safe, reliable, and low-cost in the three-phase solid state relay to solve the above problems.